Re: [PATCH 1/2] userfaultfd: remove one unnecessary warn_on in __mcopy_atomic_hugetlb

[Mike Kravetz <mike.kravetz@oracle.com>]

On 9/25/19 5:35 PM, Wei Yang wrote:
> On Wed, Sep 25, 2019 at 10:44:58AM -0700, Mike Kravetz wrote:
>> On 9/25/19 5:18 AM, Wei Yang wrote:
>>> The warning here is to make sure address(dst_addr) and length(len -
>>> copied) are huge page size aligned.
>>>
>>> While this is ensured by:
>>>
>>>     dst_start and len is huge page size aligned
>>>     dst_addr equals to dst_start and increase huge page size each time
>>>     copied increase huge page size each time
>>
>> Can we also remove the following for the same reasons?
>>
>> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
>> index 640ff2bd9a69..f82d5ec698d8 100644
>> --- a/mm/userfaultfd.c
>> +++ b/mm/userfaultfd.c
>> @@ -262,7 +262,6 @@ static __always_inline ssize_t __mcopy_atomic_hugetlb(struct mm_struct *dst_mm,
>> 		pte_t dst_pteval;
>>
>> 		BUG_ON(dst_addr >= dst_start + len);
>> -		VM_BUG_ON(dst_addr & ~huge_page_mask(h));
>>
> 
> Thanks for your comment.
> 
> It looks good, while I lack some knowledge between vma_hpagesize and
> huge_page_mask().

vma_hpagesize is just a local variable used so that repeated calls to
vma_kernel_pagesize() or huge_page_size() are not necessary.

> If they are the same, why not use the same interface for all those checks in
> this function?

If we remove the VM_BUG_ON, that is the only use of huge_page_mask() in
the function.

We can can also eliminate a call to huge_page_size() by making this change.

@@ -273,7 +272,7 @@ static __always_inline ssize_t __mcopy_atomic_hugetlb(struct mm_struct *dst_mm,
 		mutex_lock(&hugetlb_fault_mutex_table[hash]);
 
 		err = -ENOMEM;
-		dst_pte = huge_pte_alloc(dst_mm, dst_addr, huge_page_size(h));
+		dst_pte = huge_pte_alloc(dst_mm, dst_addr, vma_hpagesize);
 		if (!dst_pte) {
 			mutex_unlock(&hugetlb_fault_mutex_table[hash]);
 			goto out_unlock;
-- 
Mike Kravetz